1. Field of the Invention
The present invention relates to a carburetor for supplying fuel to an engine, and in particular relates to a carburetor having a bypass air passage for bypassing a choke valve to supply intake air during choke application.
2. Description of the Related Art
When an engine is started up before being warmed up, it is common to close a choke valve provided in a carburetor to control the air intake and enrich a gas mixture of fuel and air. It is also known to provide a small hole in the choke valve in order to cause the engine to continue running satisfactorily right after being started; e.g., as described in Japanese Laid-open Utility Model Publication No. 62-52260.
However, while the use of a richer gas mixture is preferred when the engine is started up in cold weather, in comparatively warm weather it is not desirable to use a gas mixture having the same concentration as in cold weather to have the engine keep running right after being started; therefore, the air intake should be adjusted to provide a gas mixture whose concentration corresponds to the atmospheric temperature.
Accordingly, there has been proposed a carburetor having a manual coke valve, as described in Japanese Laid-open Utility Model Publication No. 4-65959, wherein there is provided a bypass air passage for bypassing the choke valve and supplying intake air by having an air intake passage on an upstream side and an air intake passage on a downstream side connected when the choke valve is in a fully closed position. A temperature-sensitive valve for controlling an amount of bypass air in accordance with an engine coolant temperature is provided partway along the bypass air passage.
Thus, there is provided air flow volume adjusting means incorporating a temperature-sensitive valve provided partway along a passage for bypassing the choke valve, whereby opening the valve as wide as the engine coolant temperature is high can increase the flow of bypass air during engine start-up, and the gas mixture will be more readily adjusted automatically to a concentration corresponding to the temperature during engine start-up.
However, with a scheme in which a temperature-sensitive valve is opened or closed in accordance with variations in the engine coolant temperature, a time lag tends to occur when the engine coolant temperature changes in response to the change in the ambient temperature before and after engine start-up. Therefore, a gas mixture having an adequate concentration according to the atmospheric temperature may not be provided. With a temperature-sensitive body comprising a common temperature-sensitive valve, the valve body only moves a small amount in response to temperature variation. Therefore, the temperature-sensitive valve is not readily set to a large degree of opening or closing in the bypass air passage, increasing the number of instances in which a sufficient flow volume of bypass air in cold weather cannot be adequately ensured.